Environmentally hazardous fluids such as acids, oils, and toxins which can cause serious harm to the environment often need to be pumped through fluid flow systems from one location to another. When pumping such dangerous flow materials, it is important that neither the liquid nor the gases which are often released by the liquid escape to the atmosphere or pump areas outside the desired fluid pumping path.
Heretofore, conventional mechanical seals were developed to overcome rotating pump shaft sealing problems. These prior art mechanical seals allow for a fairly secure seal against the pumped fluids so as to prevent them from leaking or escaping along the shaft of the pump. Yet, in some cases the hazardous fluid penetrates these seals when the pressure within the pump becomes too high for the seal to handle, thus allowing the fluid to escape into the surrounding environment and/or the motor area of the pump. The competing interests of maintaining an efficient pump versus operating a safe one requires appropriate balancing of the two interests, because the more fluid flowing through the pump, the higher the pressure therein. Predicting the amount of safety required can only be broadly approximated based on the type of liquid to be pumped. The more hazardous the liquid, the more secure the seal construction need be.
Another problem with the aforesaid conventional mechanical seals is that gases produced by the liquids being pumped and sealed against often escape. Conventional mechanical seals are often permeated by these vapors. One solution to this problem was the creation of an arrangement known as a double seal with barrier fluid protection. In this arrangement, two seals form a cavity which is filled with a clean or environmentally safe fluid. The seal facing the excess hazardous liquid (i.e. the first seal), that which does not exit the pump where desired, inhibits movement of the liquid sufficiently to prevent penetration of the seal by the liquid. The vapor which permeates the first seal is stopped by the barrier fluid disposed in the cavity.
A drawback associated with conventional double seal systems is that any failure by the first seal can defeat the entire double seal arrangement. If the first of the two seals breaks down, the barrier fluid is permitted to escape from the cavity in effect allowing the harmful gases to penetrate the second seal thus reaching the surrounding environment. Furthermore, the harmful liquid, after the break-down of the first seal, often penetrates the second seal thus creating both gaseous and liquid leakage. These leakages typically ruin the motor which drives the pump and pollute the surrounding environment.
The breaking of the aforesaid double seals is a problem of longstanding concern due to the fact that the barrier fluid in the cavity must be maintained at a relatively high barrier pressure in order to be effective. These high pressures within the cavity often result in a break or leak in one of the two seals.
Another solution to the problem of hazardous materials leaking into the environment was the development of pumps in which the motor is disposed entirely within the pump housing. One such type is known as the "canned motor pump." A drawback of such pumps is the maintenance requirements associated with the motor. Unfortunately, the pump housing must be disassembled each time the motor requires maintenance. Furthermore, corrosive fluids being pumped often leak into and adversely affect the motor. As a result, the bearings of the motor as well as other parts thereof often clog which in effect increases the downtime of the system. In addition, the "canned motor pump" is not desireable for use with hot or dirty liquids due to the effect they have on moving parts of the motor when they leak thereinto. Finally, the efficiency of "canned motor pumps" is questionable because the motor's rotating parts often turn within corrosive fluids which have leaked into and flooded the motor area of the pump housing thus resulting in higher than normal friction forces occurring. In order to overcome this problem, sleeve bearings often have to be used instead of ball bearings.
The use of magnetic pumps was also an attempt to solve many of the aforesaid problems by housing the pump entirely within a single body (or housing) and driving the pump by a motor surrounding the body thereby preventing leakage of the fluid being pumped from affecting the performance of the motor. The motor and pump shafts in such pumps are magnetically coupled. One magnet is affixed to the motor and a magnet of opposite polarity is attached to the pump shaft within the body. These magnetic pumps, however, experience the same problems as the canned motor pumps with respect to the pump shaft bearings being exposed to the corrosive fluids being pumped. Furthermore, the efficiency of magnetic pumps can be quite low due to the loss of energy in transferring the driving force magnetically through the pair of housings to the pump shaft.
Each of the aforesaid described prior art pumps requires a high amount of maintenance if the pumped fluid is to be kept from leaking or escaping into the surrounding environment and the pump motor is to be maintained in proper operating condition.
FIG. 1 is a longitudinal partial cross-sectional view of another prior art seal construction for a pump 20 disclosed in commonly owned related U.S. Pat. No. 5,261,676, the disclosure of which is hereby incorporated herein by reference. This seal construction qualifies as prior art to the instant application because it was offered for sale more than one year prior to the instant filing date.
Prior art pump 20 includes an electric motor 1 enclosed in casing 3, the motor having a drive shaft 5 affixed to pump shaft 7 as by a key-lock mechanism. Shaft 7 of the pump is connected at one axial end to motor shaft 5 and at the other axial end to impeller assembly 9 of pump 20.
As shown, pump 20 includes three different barrier sealing devices including repeller assembly 11, triplex seal construction 13, and piston seal arrangement 15. Each of these barrier sealing mechanisms is coaxial with pump shaft 7 and is arranged axially between motor 1 and impeller assembly 9 for the purpose of preventing the fluid to be sealed against from leaking from the pump passageways axially backward toward motor 1. The aforesaid barrier sealing mechanisms function to control or seal the fluid (liquid or gas) being pumped from undesired contact with both motor 1 and the environment in which pump 20 is operating. Repeller assembly 11 and impeller assembly 9 define the wet end of pump 20, while motor 1 defines the dry end of the pump.
While most of the fluid being pumped by impeller 23 will exit the pump via volute out-flow (i.e. discharge) path 21, inevitably some of the fluid will pass axially behind impeller 23 into a narrow passageway extending from volute 25 axially backward toward and communicating with fluid chamber reservoir 35. This passageway is formed by the combination of impeller 23 and repeller assembly 11 including their plurality of inwardly extending, circular coaxial stationary back plates 29, 31, and 33.
When this inevitable portion of the fluid to be sealed against enters the aforesaid passageway, the centrifugal force exerted by the rotation of impeller 23 forces the fluid axially backward through the passageway to chamber 35 where it encounters triplex seal 13.
FIG. 2 is a longitudinal cross-sectional view of the prior art triplex seal construction 13 of pump 20. After the fluid being pumped and to be sealed against enters chamber 35, it encounters triplex seal arrangement 13. The triplex seal includes a circular rotating flange 37 which has a frontal radially extending surface 38 facing the wet end of pump 20, and a rear side 39 defining a rotating sealing surface. Flange 37 is coaxially affixed to pump shaft 7 and rotates therewith along with the impeller and repeller. The rear side or surface 39 of flange 37 includes a plurality of annular sealing members 41, 43, and 45 with sealing surfaces 47-49 defined thereon. Sealing surfaces 47-49 of flange 37 sealingly engage a plurality of stationary annular sealing or engaging members 50-52. Springs 57, 55, and 53 constantly bias stationary sealing members 50-52, respectively, into sealing engagement with rotating sealing surfaces 47-49 of flange 37.
The fluid to be sealed against enters chamber 35 by way of the aforesaid passageway and proceeds over and around the front face 38 of rotating flange 37 to the radially outer edge of rotating sealing surface 39. The fluid to be sealed against then proceeds into and through neck area 59 where the fluid applies a radially inward directed pressure on flexible annular diaphragm 61. Radially inward flexing of diaphragm 61 is limited by shelf 63, while radially outward flexing of the diaphragm is limited by a similar shelf 65 affixed to mounting flange 67. Mounting flange 67 is stationary and fixed against rotational movement as by being mounted to circular housing 69.
During operation of the pump, flexible rubber diaphragm 61 has numerous possible positions. The radially inward pressure exerted by the fluid to be sealed against on the diaphragm determines the amount of axial force placed on sealing surfaces 47-49 of flange 37 by the front engaging surfaces of stationary sealing members 50-52. Diaphragm 61 is attached across neck 59 (or fluid passageway) in such a way that any increase in pressure by the fluid to be sealed against upon the radially outer surface of diaphragm 61 will cause the diaphragm to flex radially inward thereby increasing the surface area of stationary member 71 exposed to the forward directed axial force of the diaphragm. The net effect of the radially inward flexing of diaphragm 61 is to further urge the sealing surface of stationary sealing member 50 into sealing engagement with sealing surface 47 of flange sealing member 41. In other words, the more fluid present in chamber 35, the tighter the sealing engagement between engaging members 50 and 41. Diaphragms 85 and 87 function in similar fashions.
Chambers 77 and 79 are formed between stationary annular sealing members 50-52 and may be filled with environmentally safe gas or liquid (i.e. fluid) as desired through injection orifices 81 and 83. Orifices 81 and 83 communicate with chambers 77 and 79, respectively, so as to selectively provide thereinto an environmentally safe barrier fluid at a pressure which may be used to control the amount and direction of flexing of diaphragms 61, 85, and 87. Often, only one of chambers 77 or 79 will be filled with such a barrier fluid other than that being pumped. The barrier fluid, in chamber 77 for example, applies pressure to the adjacent diaphragms (i.e. 61 and 85) in order to counteract or at least partially offset the effect of the pressure exerted by the fluid to be sealed against present in chamber 35. In such a manner, the sealing force between sealing members 41 and 50 may also be controlled by the pressure of the barrier fluid disposed within chamber 77.
Additionally, if the fluid to be sealed against somehow leaks through the sealing interface between rotating seal surface 47 and stationary member 50, and into chamber 77, chamber 79 may be filled with a barrier fluid so as to offset the pressure of the fluid to be sealed against upon the radially outer surface of diaphrapm 85.
While the aforesaid triplex seal construction provides excellent results, it is felt that the three annular sealing interfaces could be improved regarding their sensitivity to temperature and pressure. Seal faces 47 and 50 are held to flatness tolerances of about two helium light bands (0.000023 inches) or less. Therefore, as in virtually all mechanical seals, small changes in temperature or pressure can minimize the effectiveness of the flatness of the sealing interface between rotating sealing surfaces 47-49 and stationary sealing members 50-52 of triplex seal 13. For example, changes in temperature may cause the sealing surfaces to expand or contract thereby eventually creating a small gap through which the fluid to be sealed against can leak.
Another area for improvement of the aforesaid triplex seal 13 is its resistance to vibration. The effects of pump shaft 7 vibration on the seal are highest with respect to the most radially outward seal made up of stationary member 50 and rotating sealing surface 47. The fact that this sealing interface is positioned furthest radially outward from pump shaft 7 makes it the most sensitive to vibration induced by the shaft. The most radially inward seal made up of stationary member 52 and rotating surface 49 is of course less sensitive to pump shaft vibration due to its radial position. As a result, any internal or external vibration will have the most effect on the outermost seal.
It is apparent from the above that there exists a need in the art for a seal construction for a pump which is even less sensitive to surrounding changes in temperature, pressure, and vibration.
It is a purpose of this invention to fulfill the above-described needs, as well as other needs apparent to the skilled artisan from the following detailed description of this invention.